1. Field of the Invention
This invention relates to a method for determining an image point in an object image on the basis of an image signal comprising image signal components representing image information at respective picture elements on a recording medium on which a radiation image including the object image has been recorded.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. Novel radiation image recording and reproducing systems which accurately detect an image signal have been proposed in, for example, U.S. Pat. No. 4,527,060. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order approximately to ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read out means. For example, the term "read out conditions" may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed a which the stimulable phosphor sheet is scanned with the light beam ma be changed.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary read-out image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
Various methods have been proposed for calculating how the read-out conditions for the final readout and/or the image processing conditions should be adjusted on the basis of an analysis of the image signal (including the preliminary read-out image signal). As one of such methods, it has been proposed in, for example, U.S. Pat. No. 4,682,028 to create a histogram of the image signal. When a histogram of an image signal is created, the characteristics of the corresponding radiation image recorded on a recording medium such as a stimulable phosphor sheet or X-ray film can be ascertained based on, for example, the maximum value of the image signal, the minimum value of the image signal, or the value of the image signal at which the histogram is maximum, i.e. the value which occurs most frequently. Therefore, if the read-out conditions for the final readout, such as the read-out gain or the scale factor, and/or the image processing conditions are based on an analysis of the histogram of the image signal, it becomes possible to reproduce a visible radiation image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
Also, in the course of recording a radiation image, it is often desirable for portions of the object not related to a diagnosis or the like to be prevented from being exposed to radiation. Further, when the object portions not related to a diagnosis or the like are exposed to radiation, the radiation is scattered by such portions to the portion that is related to a diagnosis or the like, and the image quality is adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium an irradiation field stop is often used to limit the irradiation field to an area smaller than the overall recording region of the recording medium so that radiation is irradiated only to that portion of the object, which is to be viewed, and part of the recording medium.
However, in cases where the read-out conditions for the final readout and/or the image processing conditions are calculated on the basis of the results of an analysis of the image signal in the manner described above and the image signal is detected from a recording medium, on which the irradiation field was limited during the recording of the radiation image, the radiation image cannot be ascertained accurately if the image signal is analyzed without the shape and location of the irradiation field being taken into consideration. As a result, incorrect read-out conditions and/or an incorrect image processing conditions are set, and it becomes impossible to reproduce a visible radiation image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In order to eliminate the aforesaid problem, it is necessary to determine the shape and location of an irradiation field and then to calculate the read-out conditions for the final readout and/or the image processing conditions on the basis of only the image signal representing image information stored in the region inside of the irradiation field.
Accordingly, the applicant has proposed in, for example, U.S. patent application No. 182,685 a novel method for accurately determining the shape and location of an irradiation field even when the irradiation field has an irregular shape. The proposed method comprises the steps of detecting a contour point, which is considered to be present on the contour of the irradiation field, on each of a plurality of radial lines each of which connects a predetermined point located in the region inside of the irradiation field with an edge of a recording medium, and determining that the region surrounded by lines connecting the thus detected contour points is the irradiation field.
In the proposed method for determining the shape and location of an irradiation field, the aforesaid predetermined point should be located in the region inside of the irradiation field, and should preferably be located in an object image, which is formed in the region inside of the irradiation field. In cases where the recording of a radiation image is carried out with an irradiation field stop, since the irradiation field stop is used to limit the irradiation field so that an image of only that portion of the object which is to be viewed is recorded, the image points in the region inside of the irradiation field (particularly, the image points at positions neighboring the center point of the irradiation field) are generally located in the region inside of the object image.
In cases where the shape and location of the irradiation field are first determined and then the image signal representing the image information recorded in the region inside of the detected irradiation field is analyzed in the manner as that described above, appropriate read-out conditions for the final readout and/or appropriate image processing conditions can be determined.
However, in order to determine the shape and location of an irradiation field with the method proposed in U.S. patent application No. 182,685, it is necessary to find an image point located in the region inside of the irradiation field (preferably, an image point located in the region inside of the object image).
One approach is to determine the center point of the recording medium as the image point located in the region inside of the irradiation field (an image point located in the region inside of the object image). For example, as shown in FIG. 5A, in cases where an irradiation field 2 is present at the center portion of a recording medium 1, the center point C of the recording medium 1 is one of the image points located in the region inside of the irradiation field 2. In most cases, the center point C of the recording medium 1 is one of the image points located in the region inside of an object image 3.
However, as shown in FIG. 5B, in cases where the irradiation field is deviated from the center point C of the recording medium 1, an image point in the region inside of the irradiation field 2 cannot be found with the aforesaid method. Also, as shown in FIG. 5C, in cases where the image recording was carried out without the irradiation field stop being used, all positions on the recording medium 1 including the center point C of the recording medium 1 are the image points located in the region inside of the irradiation field 2. However, in this case, no irradiation field stop was used, and the probability of the object image 3 being deviated from approximately the center point of the irradiation field 2 (which center point coincides with the center point C of the recording medium 1 in this example) is higher than when the image recording was carried out with the irradiation field being limited as shown in FIGS. 5A and 5B. Therefore, the center point C of the recording medium does not necessarily constitute one of the image points located in the region inside of the object image.
The image point located in the region inside of the object image should be determined when the shape and location of the irradiation field are to be recognized as described above, and when which position on a radiation image is to be employed as the center point of a visible image is determined in cases where, for example, part of the radiation image is enlarged and reproduced into the visible image.